The use of personal wireless radio communications systems, such as digital cellular telephones, operating in frequency bands between 900 MHZ and 5800 MHZ, is expanding at a fast rate. One of the key issues in the success of wireless hardware is the overall size and weight of hand-held battery operated devices. Much of the bulk of such devices is contributed by a power source, typically a battery pack. If the size of the battery pack in the enclosure of the hand-held device could be reduced, the volume saved could be used to implement other features or functions without increasing the overall size of the device and would probably also result in a decrease in the weight of the device. The size of the battery pack is dictated mainly by the operating voltage of the wireless terminal. In order to reduce the size and weight, it is desirable to reduce the required supply voltage of active circuitry used in such devices.
In many communications systems, specifically wireless communications systems, an information signal is modulated onto a higher frequency carrier before being transmitted. In these systems, there is often a need at the receiver end for some form of frequency translation back to lower frequencies. A decision device is then used to recover the information contained in the lower frequency signal. The frequency translation is often performed using frequency mixers.
Frequency mixers are important building blocks in transceiver design since the dynamic range and noise/distortion performance of a transmitter/receiver is determined, and often limited, by the first up-down-conversion mixer.
The design of mixers forces many compromises between conversion gain, local oscillator performance, linearity, noise figure, port-to-port isolation, voltage supply and power consumption. There are several types of mixer design used in the industry, probably one of the more popular being the Gilbert mixer. The Gilbert mixer has a structure which is based on the application of the well-known cascode structure (also known as a totem pole or tree structure), which, although providing a stable and approximately linear performance, has a disadvantage of requiring a relatively high voltage to operate.